Though there are several reasons why you would be able to get into the parks early (dining reservations, etc.) most of the parks actually open the gates earlier than their posted times. Although, they may restrict which areas of each park you can get to before the actual opening time.
We like to be at the gate 30-45 minutes early sometimes earlier. You’ll at least be able to get to do some early shopping or get in line at Starbucks early. During our most recent trip, we got to Hollywood Studios an hour early and the line for Jedi Training was already open. So, get there sooner!